The present invention relates to a ballast for a lamp having an inverter which comprises at least one switch, and having a drive circuit for alternately opening and closing the at least one switch, and to a method for operating such a ballast.
Prior art
In ballasts, which are controlled directly by a microcontroller without any further signal conversion, the possible operating frequencies cannot be chosen in indefinitely fine steps. Due to the restrictions of the hardware, in which particular mention should be made of the register width and processor clock, only a specific fixed number of possible operating frequencies can be produced. When changing from one discrete frequency to the next, for example as a result of control processes, this results in abrupt brightness changes of the connected lamp, which may be regarded as being disturbing.
One possible way to counteract this is to increase the clock frequency of the processor and to use broader registers to refine the steps such that the remaining sudden brightness changes are no longer conspicuous. However, this is generally undesirable for cost reasons. Typical solutions for this problem therefore generally provide for the signal supply from the microcontroller to be converted from digital to analogue form. In this case, the output signal from the processor is normally integrated using a capacitor. The resultant voltage then controls an oscillator, for example a VCO (Voltage Controlled Oscillator). The frequency of the VCO can be adjusted in very fine steps. However, the costs incurred here are also still considerable.
The object of the present invention is therefore to develop a ballast of the type mentioned initially, as well as the method mentioned initially for operating a ballast, such that abrupt brightness changes of a connected lamp can be avoided in a cost-effective manner.
This object is achieved by a ballast having the features of patent claim 1, and by a method for operating a ballast having the features of patent claim 8.
The invention is based on the knowledge that the integration of the changing light brightness is not carried out using a capacitor provided for this purpose, but by the inertia of human perception. A periodically fluctuating lamp brightness is not perceived if the fluctuation frequency is more than 80 Hz. Any perceivable brightness between the fixed predetermined discrete values can be produced by switching between at least two different frequencies.
The drive circuit is preferably designed to carry out a change between the different drive frequencies at a switching frequency which is chosen such that the different lamp brightnesses, which are correlated with the different drive frequencies, can no longer be resolved by a human eye. The switching frequency is thus preferably at least 60 Hz, and in particular at least 80 Hz.
A mains rectifier may be connected upstream of the inverter, and may be fed from an AC voltage mains system which is at a mains frequency, with the switching frequency being synchronized to the mains frequency. This is because, if ripple occurs at the mains frequency or at multiples of it in the ballast, low beat frequencies can occur if the drive is not synchronized, whose frequency may be so low that these frequencies can once again be perceived by the human eye. By way of example, the mains frequency is 50 Hz or 60 Hz, and the corresponding switching frequencies are then multiples of 50 Hz, in particular 100 Hz, or multiples of 60 Hz, in particular 120 Hz. Alternatively, it is also possible to provide for the mains frequency to be a multiple of the switching frequency.
When the ballast is operated from a DC voltage source, for example from batteries, no synchronization is required with externally produced frequencies. In this case, a switching frequency is chosen which can be implemented well technically and is at the same time greater than the flicker frequency which can still be perceived by the human eye.
Further advantageous developments of the invention are defined in the dependent claims.